The present invention generally relates to an optical communication network, and more particularly, to a system and method for reducing four-wave mixing along an extended optical fiber link.
Optical communication networks serve to transport information at high data rates between a number of physical sites, commonly referred to as nodes. Each of these nodes are interconnected with the various other nodes by information conduits, commonly referred to as links. These links are comprised of at least one, and usually several, optical fibers. Information is usually presented to the optical communication network in the form of time-domain electrical data signals, and may represent any combination of telephony, video, or computer data in a variety of formats.
As depicted in FIG. 1, multiple data signals D1, D2, D3 and D4 are typically provided to a site for transport over an optical network 10. Typically, these data signals are coupled via a digital cross connect switch (not shown) to an optical transmitter. Each optical transmitter includes at least one, and usually several semiconductor lasers. Each semiconductor laser responsively emits light that is intensity-modulated by a corresponding input electrical data signal. This intensity-modulated light is wavelength division multiplexed (WDM) and transmitted over an optical fiber 14 to an optical receiver 16 at a remote site. That is, the transmitter converts the electrical data signals to optical signals each having a carrier frequency in the light spectrum, each input electrical data signal modulating a light carrier having a different light frequency and corresponding wavelength. Thus, as shown, for four electrical input data signals, there are transmitted four optical signals having four different wavelengths across a single optical fiber. In this manner, the data from each of these data signals is wavelength division multiplexed (WDM) over a single optical fiber.
Examples of these electrical data signals can be SONET-compliant STS-48 or STS-192 synchronous data signals each bearing digital data at about 2.5 Gbps or 9.9 Gbps, respectively. Correspondingly, the optical data signals may include SONET OC-48 or OC-192 signals bearing digital data at approximately 2.5 or 9.9 Gbps respectively. Generally, various high data rate electrical data signals are multiplexed over a single optical medium.
Frequently, optical fiber lengths between nodes are so long that several intermediate amplification stages 18 are required along the length of the fiber 14, as shown in FIG. 1. In addition to intensity loss along the fiber, the fiber medium can also introduce other impairments, such as chromatic dispersion. Chromatic dispersion causes individual light pulses to become blurred and less discernible along the optical fiber. If chromatic dispersion is not compensated in a distributed fashion, the total dispersion will accumulate along the length of the fiber as shown at 20 in FIG. 2, and the optical signal may eventually be unrecoverable at the receiving end.
One common practice for minimizing chromatic dispersion is to incorporate a dispersion compensator (DC) into each line amplifier 26 of a network 24 as shown in FIG. 4. In this approach, dispersion compensation is accomplished in a distributed fashion, as shown at 28 in FIG. 5. At each optical amplifier 26, the dispersion compensator attempts to minimize the chromatic dispersion at that point, as shown at P. The dispersion compensator nearly cancels the chromatic dispersion introduced by the optic fiber 14 up to that point. Furthermore, the dispersion compensator in a WDM system attempts to minimize the chromatic dispersion across a band of wavelengths by exhibiting a dispersion slope characteristic, as a function of wavelength, that is opposite that of the native optic fiber. Conventionally, each dispersion compensator is set to reduce chromatic dispersion to near zero at the amplifier for all wavelengths in a band.
Another potential transmission impairment observed along optical lengths is mixing among optical carriers, as shown at 22 in FIG. 3 and at 29 in FIG. 6. This mixing is caused by non-linearities in the amplification stages, and in the fiber medium at high carrier power levels. Relatively high launch powers into an optical fiber can cause the optical carriers to mix and create unwanted signal components that interfere with desired carriers. This mixing problem can be circumvented by lowering carrier power, and by steering the optical carrier wavelengths so as to move the unwanted byproducts into a harmless position in the receive spectrum. However, using lower transmit power limits the maximum distance between optical amplifiers. Steering of optical wavelengths complicates the control and selection of the optical carriers, and subverts the desire to use an evenly-spaced comb reference in a dense WDM channel plan.
There is a desire to provide an improved optical communications link that offers improved tolerance to high launch power of an optical carrier into an optical fiber to overcome attenuation through the length of the optical fiber, while minimizing the carrier interactions such as four-wave mixing that usually accompany such high launch power levels.
The present invention achieves technical advantages as an optical communication link with improved tolerance to high launch power of optical carriers and reduced carrier mixing. In accordance with the present invention, the optical communication link includes an optical fiber, at least one optical amplifier disposed along the optical fiber, and at least one dispersion compensator. In one preferred embodiment, a first dispersion compensator is located along an optical fiber between a transmitter and a first optical amplifier, with first dispersion compensator placed at some substantial distance prior to the first optical amplifier. This allows an optical carrier to be launched at a relatively high power into a fiber under dispersive conditions. A second dispersion compensator may be inserted into the fiber line at a point after the first optical amplifier where the high launch power has been eventually diminished by fiber attenuation and non-linearity of the fiber medium is no longer a problem. The second dispersion compensator brings the dispersion to a manageable value. The loss introduced by the dispersion compensator is tolerated by an ample gain budget. The optical signal then continues towards a second optical amplifier and the dispersion is again allowed to accumulate so that the optical signal is dispersively propagated during and immediately after amplification at the second optical amplifier. In this first preferred embodiment, the amplifiers and dispersion compensators are alternately spaced from one another. Four-wave mixing is reduced by advantageously using the property that chromatic dispersion in the optical fiber tends to disrupt the phase coherence that is required for mixing to occur. The present invention provides manageable chromatic dispersion at the amplifiers which tends to reduce the four-wave mixing that can occur through the amplification stage when multiple wavelengths are amplified.
According to a first method of the present invention, there is provided a method for processing an optical signal along an optical fiber link. The method comprises coupling an optical signal to the link, dispersion compensating the optical signal, allowing the optical signal to become mildly dispersed as it travels through the fiber, and then amplifying the mildly dispersed optical signal using an optical amplifier. The optical amplifier amplifies a moderately dispersed optical carrier, which advantageously reduces four-wave mixing. Consequently, a relatively high launch power of the optical carrier into the optical fiber is allowed. Another dispersion compensator compensates for the accumulated chromatic dispersion at a point past the optical amplifier, after sufficient attenuation has occurred due to fiber loss such that power level is insufficient to cause non-linearities that lead to mixing.
According to a second preferred embodiment of the present invention, there is provided an optical communication link wherein the dispersion compensator is collocated with, but prior to, a high-power output stage of an amplifier, wherein the dispersion compensator over-compensates both a slope and an absolute dispersion at the point in the fiber where the dispersion compensator is located. This assures that the high power amplification stage launches a high power carrier into the optical fiber under dispersive conditions thus avoiding four-wave mixing. According to this second preferred embodiment, the optical amplifier is provided with a pre-amplifier stage and a high power output stage, wherein the is serially coupled between the pre-amplifier stage and the output stage. The pre-amplifier stage runs at a relatively low output power since the carrier signal at this point is non-dispersive. Both the pre-amplifier and the dispersion compensator are operated at an acceptable level to avoid driving the fiber into non-linearity which would otherwise cause carrier intermodulation.
According to a second method of the present invention, there is provided a method comprising the steps of coupling an optical signal to an optical link, dispersion compensating the optical signal using a dispersion compensator, wherein the dispersion compensator is collocated with the optical amplifier, and the dispersion compensator overcompensates both a slope and an absolute dispersion of the fiber such that the amplifier launches the optical signal into a dispersive condition.
According to a third embodiment of the present invention, an optical communication link is provided with a forward dispersion compensator to overcompensate chromatic dispersion at this point and ensure that there is a neutral dispersion signal upon arrival at the next optical line element or receiver. Each subsequent amplifier is then also provided with a dispersion compensator to overcompensate both the slope and absolute dispersion of the fiber at that point to ensure that a neutral dispersion signal arrives at the next optical line element or receiver. This allows the amplification stages to launch the optical carrier into the fiber link at a high power under dispersive conditions thus avoiding four-wave mixing.
According to a third method of the present invention, the dispersion compensators render a neutral dispersion signal upon arrival at the next optical line element. The dispersion compensators are collocated with the optical amplifiers, and overcompensate both the slope and absolute dispersion of the fiber to render a dispersive condition for the amplifiers. This allows the amplification stages to launch the optical carrier at a high power into the fiber link under dispersive conditions thus avoiding four-wave mixing.